Building an API with AWS Lambda and API gateway

Create a Serverless API with AWS Lambda and API Gateway tutorial

Posted on September 4, 2020

Why use Lambda/API gateway

  • It’s serverless, so no having to provision servers and keep them updated.
  • Security – You can give Lambda function access to only the services that it needs.
  • It makes your api easy to manage.
  • It’s easy to add authentication with Cognito.
  • Built in versioning.
  • Quick to build and deploy
  • You only pay for what you use. You are charged based on the number of requests for your functions and the duration, the time it takes for your code to execute. The AWS Lambda free usage tier includes 1M free requests per month and 400,000 GB-seconds of compute time per month.

Create a Lambda function

Login to your AWS account and navigate to the Lambda console
Click Create function, select ‘Author from scratch‘, give your function a name and click ‘Create Function

When you create the function, AWS will create role which we can update at a later date to give it access to other services such as DynamoDB.

Now update your function code with the following so we can separate out GET and POSTS requests. Don’t forget to click ‘Save’ in the top right.

exports.handler = async (event) => {
    const method = event.httpMethod;
    let response;
    if (method === 'GET') {
        response = {
            statusCode: 200,
            body: JSON.stringify('GET success!'),
    } else if(method === 'POST') {
        response = {
            statusCode: 200,
            body: JSON.stringify('POST success!'),
    return response;

There seems to a bit of a discussion at the moment on whether it’s better to create separate functions for each endpoint or keep it all in one function. I personally think this depends on the size of your application.

The main thing is ensuring your code is easy to find and update.

This is a very small API with just the one endpoint, so we’ll just be creating the one function. 

Create an API gateway (REST API)

Next we need to create the API gateway.

Click ‘Add Trigger‘ in the designer block of the lambda function.
Select API Gateway wit the following settings and click ‘ADD’

  • Create an API
  • Security Open (we’ll cover securing your API Gateway in a future article)

We’ve now created a API gateway to trigger your lambda function.

The next thing to do is create a GET and POST method and publish the API.

  • Click on the gateway name in the ‘API Gateway’ block’
  • Click ‘Actions’ > ‘Create Method’ and choose ‘GET’ in the drop down menu that appears.
  • Check/select ‘Use Lambda Proxy integration’
  • Add the Lambda functions name to the form and click save.

Repeat this but for the POST method.

We now need to deploy the API by clicking on Actions > Deploy API > deployment stage = default > deploy.

If you navigate back to your Lambda function and click on API Gateway trigger block, you’ll see your methods appear with with the relevant endpoints.

Test your API with postman

I use Postman for testing my APIs which can be downloaded here:

Copy the API endpoint from the relevant method in the ‘API Gateway’ block on the Lambda function screen, add it postman and click send. You should see ‘GET success!’ or ‘ ‘POST success!’ in the response depending on which method you’ve called.

And there you have it – your first Serverless API built with AWS Lambda and API gateway


This is obviously a very simple API, but it gives you the gist of how quick and easy they are to create and deploy.

I’ll cover versioning, CORS, database integration and securing your API with Cognito in a future article.